This invention relates to a method for brazing aluminium or alminium alloy parts, employing a brazing solder and flux. More particularly, this invention relates to such method, in which the flux itself would not be oxidized in a brazing atmosphere so that its amount could be at a minimum amount and so that it should neither pollute the brazing atmosphere and nor erode furnace structures surrounding the brazing atmosphere. This invention relates also to a continuous type of furnace which could advantageously be employed for successively brazing aluminium or alminium alloy parts.
Modern fluxes such as metallic fluorides which are disclosed in U.S. Pat. No. 4,475,960 of Yamawaki et al. and metallic chlorides mixed with metallic fluorides which are disclosed in U.S. Pat. No. 4,643,348 of Takahashi, are widely used in the brazing of aluminum parts. As they are so efficient to achieve their works for dissolving filmy oxides from alminium surfaces and removing the oxides from the surfaces and accordingly for allowing brazing solders to flow freely on and into the surfaces, their amount of use is minimized, whereby aluminium brazing costs become low. This contributes also to the prevention of pollution problems, because an amount of water for washing off fluxes after a brazing operation is minimized, and sometimes a washing step is eliminated in the brazing operation.
In order to have such fluxes which are used at a minimum amount exert their full capacities without the oxidation of fluxes themselves, it has been common and inevitable for brazing aluminium parts to employ as a brazing chamber such a metallic muffle as disclosed in U.S. Pat. No. 4,416,623 of Takahashi, which is hermetically filled with an inert atmosphere such as nitrogen, and in which the parts to be brazed are heated with a solder and flux applied on them. A heating temperature by which the aluminium or aluminium alloy parts are brazed, is 580.degree. C.-660.degree. C., since a melting point of the fluxes mentioned above is around about 550.degree. C. and the flux most lately developed has a melting point of 500.degree. C., and since Al-Si system solders which are generally employed for aluminium brazing together with the above-mentioned fluxes, have a melting point between 500.degree. C. and 630.degree. C.
From a thermal aspect, there is no problem at all to use a metallic muffle under such comparatively low heating temperature, as metals which form the muffle can stand well such temperature, although it has been noticed by the present inventor as drawbacks that fluxes employed in brazing aluminium parts break oxidized surface films of heat and rust-resisting steel of the metallic muffle too, when they are heated, decomposed, and liquidized or evaporated, and they finally spot the steel, whereby an inert atmosphere within the muffle is polluted. Partly because that such drawbacks have been taken unavoidable on the whole, and partly because that differently from and compared to other metals such as steel or Cu-base or Fe-base alloy containing a small amount of Al, aluminium and its alloy matrix of which is Al seemed too sensitive to heat-treatment within a muffle made from materials other than metallic materials, nobody has touched, in brazing aluminum parts, to replace the metallic muffle with one made from other materials, except prior cases such as disclosed in Japanese post-examination patent publication No. 7-60063 of Takahashi and in U.S. Pat. No. 5,538,177 of Takahashi. In the example of former case, steel is heated at a high temperature in a muffle case made from carbon or graphite solely for the purpose of hardening it without oxidation and decarbonization. And, in the examples of latter case, copper or iron-base alloys are welded at a very high temperature in a batch type chamber having carbon walls. However, in this latter case too, nothing was taught about damages the chamber will suffer on account of fluxes if it were made from metals, because such damages had not been expected there since the fluxes were used at a negligible amount as they intended to clean only Al constituent parts of a low percentage, since they were not required to render the alloys receptive to amalgamation with a solder as no solder was used there, and since decomposed and vaporized fluxes should not remain in the chamber long as the atmosphere in the chamber was changed repeatedly at each batch cycles.